Precipitated silica

ABSTRACT

The precipitated silica has the following physico-chemical parameters: 
     
       
         
               
               
               
             
                   
                   
               
                   
                 BET surface area (DIN 66131) in m 2 /g 
                 400-600 
               
                   
                 DBP index (DIN 53601) in g/100 g 
                 300-360 
               
                   
                 Compacted density (DIN 53194) in g/l 
                  70-140 
               
                   
                 Grindometer value (ISO 1524) in μm 
                 15-50 
               
                   
                 Size distribution index I 
                 &lt;1.0 
               
                   
                 measured with a Malvern instrument 
               
                   
                   
               
                   
                 
                   
                     
                       
                         
                           Size 
                            
                           
                               
                           
                            
                           distribution 
                            
                           
                               
                           
                            
                           index 
                            
                           
                               
                           
                            
                           I 
                         
                         = 
                         
                           
                             
                               d 
                               90 
                             
                             - 
                             
                               d 
                               10 
                             
                           
                           
                             2 
                              
                             
                               d 
                               50 
                             
                           
                         
                       
                     
                     
                     
                         
                     
                   
                 
               
                   
                   
               
           
              
             
             
              
              
              
              
              
              
              
              
              
             
          
         
       
     
     This precipitated silica is prepared by milling a precipitated silica in accordance with DE-A 31 44 299 in a classifier mill or a fluidized bed counter-flow mill. A polyethylene wax emulsion may be added before the milling procedure. The precipitated silica then has the following physico-chemical parameters: 
     
       
         
               
               
               
             
                   
                   
               
                   
                 BET surface area (DIN 66131) in m 2 /g 
                 351-600 
               
                   
                 DBP index (DIN 53601) as a % 
                 300-360 
               
                   
                 Carbon content as a % 
                 1-8 
               
                   
                 Compacted density (DIN 53194) in g/l 
                  70-140 
               
                   
                 Grindometer value (ISO 1524) in μm 
                 15-50 
               
                   
                 Size distribution index I 
                 &lt;1.0 
               
                   
                   
               
           
              
             
             
              
              
              
              
              
              
              
             
          
         
       
     
     The precipitated silicas may be used as matting agents in lacquer systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 09/208,454, filed Dec. 10, 1998 pending.

This application is a continuation of U.S. provisional application Ser. No. 60/070,191, filed Dec. 30, 1997, and German patent application No. DE 19755287.0, filed Dec. 12, 1997, the entire disclosures of these applications being incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a precipitated silica, a process for its preparation and its use as a matting agent.

BACKGROUND OF THE INVENTION

It is known that synthetic, precipitated silicas or silica gels can be used as matting agents (DE-PS 24 14 478, DE-PS 17 67 332, DE-OS 16 69 123, DE-AS 15 92 865, DE-A 38 15 670).

The matting power of a silica depends on a variety of factors, such as, for example, the type of silica, the particle size, the particle size distribution, the refractive index and also the lacquer system. The shape and size distribution of secondary particles in the silica are of particular importance.

In addition to being very efficient, expressed by the reduction in degree of gloss as compared with the non-matted lacquer film, a silica which is used as a matting agent also has to satisfy a number of other requirements. Thus, for example, there should be no undue thickening of the lacquer system due to the silica which is introduced. A smooth surf ace to the lacquer should be produced on the corresponding thin lacquer coatings. Specks which have an adverse effect on the surface quality must be avoided.

The document DE-A 31 44 299 describes precipitated silicas and a process for preparing these precipitated silicas which are characterized by the following physico-chemical properties:

BET surface area according to DIN 66131 in m²/g 400-600 DBP index according to DIN 53601 as a % 320-360 and BET surface area according to DIN 66131 in m²/g 400-600 DBP index according to DIN 53601 as a % 310-360 Compacted density according to DIN 53194 in g/l  75-120 “Alpine” sieve residue > 63 μm in wt. % < 0.1

When preparing these silicas, an Alpine transverse flow mill or a jet mill is used to mill the product following spray drying. It is also specified in this document that these precipitated silicas are valuable, highly effective matting agents for lacquers. Precipitated silicas which are prepared using these types of mill lead to disadvantageous roughness of the surface due to the presence of large specks in the final lacquer. The grindometer value (according to ISO 1524) in black stoving enamel is greater than 100 μm and 85 to 90 μm respectively for the known precipitated silicas. Thus these precipitated silicas can only be used to a limited extent as matting agents.

There is now the object of preparing a precipitated silica which does not have these disadvantages.

SUMMARY OF THE INVENTION

The object of the invention is a precipitated silica which is characterized by the following physico-chemical parameters:

BET surface area according to DIN 66131 in m²/g 400-600 DBP index according to DIN 53601 in g/100 g 300-360 Compacted density according to DIN 53194 in g/l  70-140 Grindometer value according to ISO 1524 in μm 15-50 Size distribution index I <1.0 measured with a Malvern instrument ${{Size}\quad {distribution}\quad {index}\quad I} = \frac{d_{90} - d_{10}}{2d_{50}}$

Another subject of the invention is a process for preparing precipitated silicas according to the invention with the physical-chemical parameters:

BET surface area according to DIN 66131 in m²/g 400-600 DBP index according to DIN 53601 in g/100 g 300-360 Compacted density according to DIN 53194 in g/l  70-140 Grindometer value according to ISO 1524 in μm 15-50 Size distribution index I <1.0 measured with a Malvern instrument ${{Size}\quad {distribution}\quad {index}\quad I} = \frac{d_{90} - d_{10}}{2d_{50}}$

which is characterized in that a precipitated silica which has the following physico-chemical properties

BET surface area according to DIN 66131 in m²/g 400-600 DBP index according to DIN 53601 as a % 340-380 Compacted density according to DIN 53194 in g/l 180-220 “Alpine” sieve residue > 63 μm wt. %  25-60,

is milled using a classifier mill or a fluidized bed counter-flow mill.

The initial silica is described in the document DE-A 31 44 299.

In one embodiment of the invention, mentioned by way of example, a ZPS classifier mill (Zirkoplex® Alpine Aktiengesellschaft D-8900 Augsburg) or an AFG fluidized bed counter-flow mill may be used.

In another variant of the invention, the precipitated silica according to the invention may be classified after milling in order to adjust to a specific granular fraction. In a preferred embodiment of the invention, the precipitated silica has the size distribution shown in FIG. 1.

Classifying may be performed, for example, using an ATP Turboplex fine classifier (Alpine Aktiengesellschaft D-8900 Augsburg).

The invention also provides a precipitated silica coated with a polyethylene wax emulsion which is characterized by the following physico-chemical parameters:

BET surface area according to DIN 66131 in m²/g 351-600 DBP index according to DIN 53601 as a % 300-360 Carbon content as a % 1-8 Compacted density according to DIN 53194 in g/l  7-140 Grindometer value according to ISO 1524 in μm 15-50 Size distribution index I < 1.0

This precipitated silica can be prepared by adding polyethylene wax emulsion to a precipitated silica which has the following physico-chemical characteristics:

BET surface area according to DIN 66131 in m²/g 400-600 DBP index according to DIN 53601 as a % 340-380 Compacted density according to DIN 53194 in g/l 180-220 “Alpine” sieve residue > 63 μm wt. % 25-60

and then drying and milling the product using a classifier mill or a fluidized bed counter-flow mill.

In one embodiment of the invention, the precipitated silica can be prepared by liquefying the filter cake under the action of shear forces, adding polyethylene wax emulsion, spray drying and then milling using a classifier mill or a fluidized bed counter-flow mill.

A precipitated silica in accordance with DE-A 31 44 299 is preferably used as the starting silica.

Precipitated silica according to the invention has the following advantages:

The advantages of precipitated silicas according to the invention are in particular their high matting efficiency, in addition to further advantages such as the very smooth surface of the dry lacquer, high transparency and a small effect on the rheology (viscosity) of the lacquer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the size distribution of classified precipitated silica.

FIG. 2 shows the particle size distribution of precipitated silicas according to the invention compared to the particle size distribution of a precipitated silica in accordance with DE-A 31 44 299.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

A precipitated silica prepared in accordance with Example 1 from DE-A 31 44 299 is milled in a ZPS 100 Zirkoplex® classifier mill from the Alpine company, by varying the throughput and the process parameters such as speed of rotation of the classifier, milling throughput and milling air. The trial parameters, the physico-chemical data and the paint properties which are obtained in black stoving lacquer are given in Table 1.

EXAMPLE 2

A precipitated silica prepared in accordance with Example 1 from DE-A 31 44 299 is milled in an AFG 200/1 fluidized bed counter-flow mill, from the Alpine company, while varying the throughput and the process parameters such as rate of rotation of the classifier, or the milling air. The trial parameters, the physico-chemical data and the paint properties which are obtained in black stoving lacquer are given in Table 2.

EXAMPLE 3

Precipitated silicas which are prepared in accordance with Example Ic or Example 2c (see Tables 1 and 2) are classified in an ATP 50 turboflex fine classifier to give a finer and a coarser fraction. The process parameters, the physical data and the paint test results which are obtained in black stoving lacquer are given in Table 3.

EXAMPLE 4 (comparative example)

The unmilled, spray-dried silica, prepared in accordance with DE 31 44 299 (Example 6), is milled on a UP 630 Alpine transverse flow mill. The physico-chemical data and paint properties of the product obtained are given in Table 4.

EXAMPLE 5 (comparative example)

The unmilled, spray-dried silica, prepared in accordance with DE 31 44 299 (Example 9), is milled using an MC 500 Microgrinding air jet mill. The physico-chemical- data and paint properties are given in Table 4.

The effectiveness and matting efficiency of the precipitated silicas prepared according to Examples 1 to 3 are tested in a black stoving lacquer. The Lange gloss values, at angles of reflection of 60° and 85°, and the Hegman grindometer value were also assessed.

The B. Lange gloss meter was used to determine the degree of gloss, which is a measure of the matting power of the matting silica tested. The B. Lange gloss meter uses angles of incidence and reflection of 60° and 85°. The degrees of gloss measured are cited as percentages. The lower this value, the better is the matting capacity of the precipitated silica. As a result, less matting agent has to be used in order to achieve a quite specific degree of gloss or a specified matting effect.

The grindometer value is determined using a grindometer. The grindometer value, which is measured in μm (micrometers) is a measure of the largest particles which can be found after stirring the precipitated silica into the final, sprayable lacquer mixture. It can be related to the production of specks in the dry lacquer film, so undesired specks or sprayed granules can be detected using the grindometer (ISO 1524).

The quality of the lacquer film surface is determined using the scanning section method developed by the Hommelwerke company and is cited as an average roughness value (Ra) according to DIN 4768/1, DIN 4762/1E and as an average depth of roughness (RZD) according to DIN 4768/1.

The black stoving lacquer used had the following composition:

Parts by wt Carbon black paste, tack 1 8.0 Jägalyd R40, 60% strength in xylene 50.8 Maprenal MF 800, 55% strength in butanol 25.9 Baysilone paint additive OL 17,1 % in 2.0 Xylene 13.3 Thinner 100.0 Thinner Xylene 75.0 Butanol 10.0 Ethoxypropanol 15.0 100.0

4 g of precipitated silica are stirred into 100 g of lacquer with a blade stirrer at 2000 rpm for 10 minutes. The viscosity of the mixture is adjusted to a flow time of 20 seconds using xylene (DIN; 4 mm nozzle).

The lacquer is sprayed to give an approximately 30 μm thick dry layer on sheet metal, air dried and fired at 180° C. for 30 minutes.

EXAMPLE 6

The paint properties of the precipitated silicas prepared according to examples la to c, a precipitated silica prepared according to DE 38 15 670 and a commercially available product (Nipsil 1009) are tested in two other test lacquer systems.

CC lacquer Parts by wt. Alftalat AN 950, 60% in Solvesso 150/Butylglycol 29.30 Solvesso 150 2.60 Titanium dioxide Kronos 2059 33.60 Aerosil R 972 0.20 Dispersion: 40 h ball mill KU 5, 60 rpm, 4900 g Alubite beads 19 mm Alftalat AN 950, 60% in Solvesso 150/Butyl 13.00 glycol Maprenal MF 900, 100% 8.10 Maprenal MF 577, 50% in butanol 0.80 Butyl glycol 2.00 Solvesso 150 2.90 Xylene 6.70 DOW CORNING PA 57 0.60 p-Tolylsulfonic acid, 20% in butanol 0.30 Total 100.00

Before use, 3.2 g of matting agent are dispersed in 150 parts by weight of lacquer using a blade stirrer at 2000 rpm.

DD lacquer Parts by wt. CAB 381-0,5 0.3 Butyl acetate, 98% strength 11.0 Ethoxypropyl acetate 16.5 Desmophen 800 15.0 Desmophen 1100 20.0 Mowilit, 50% strength in ethyl acetate 3.0 Baysilone-lacquer additive 0.1 Xylene 34.1 Total 100.00

Firstly 0.3 parts by weight of CAB 381-0.5 are carefully dissolved in 11.0 parts by weight of butyl acetate (98.0% strength, and 16.5 parts by weight of ethoxypropyl acetate using a high speed stirrer. Then the other components are added in the sequence given above and the mixture is homogenized by stirring.

Before use, the gloss lacquer is homogenized with the blade stirrer. The matting agent (amount see table 6) is dispersed in 100 parts by weight of lacquer using a blade stirrer at 2000 rpm. After a degassing time of 15 minutes, 50 g of the hardener Desmodur L 75 are added and homogenized with the blade stirrer for 2 minutes at 1000 rpm. The mixture is applied to a thoroughly pre-cleansed glass block and to a black, high gloss, lacquered glass block using a spreader with a 200 μm slit.

The test results in CC lacquer are given in Table 5 and in DD lacquer in Table 6. For comparison the precipitated silicas according to DE 38 15 670 and the commercial product NIPSIL E 1009 are also given. A comparison of the data determined can be obtained from the tables.

FIG. 1 shows the size distribution of classified precipitated silica of Example 2c of the invention compared with comparative Example 4c, using data set forth in the Tables below.

FIG. 2 shows the particle size distribution of precipitated silica of Example 2 of the invention, compared with the particle size distribution of precipitated silicas of comparative Examples 4 and 5, according to DE 31 44 299, using data set forth in the Tables below.

TABLE 1 Speed Speed of Classifier Through- Thickness of mill classifier air put Particle size (Malvern) Grindo Gloss Roughness Viscosity of coating Ex. rpm rpm m³/h kg/h d 4.3 d 10 d 50 d 90 μm 60° 85° Sheen RZD Ra s μm 1a 10700 11000 175 10 8.34 4.48 7.03 12.89 23 23.8 72.0 48.2 2.27 0.27 36 30 1b 10000 10500 180 15 9.76 4.53 7.11 15.84 27 21.8 70.3 48.5 2.37 0.28 36 30 1c 10000  9000 200 30 9.34 4.52 8.03 13.87 28 24.7 67.9 43.2 34 28 1d 10000 10000 145 15 9.97 4.27 6.78 16.13 33 26.0 73.4 47.4 38 29

TABLE 2 Speed of Milling Through- Particle size Thickness classifier air put (Malvern) (μm) Grindo Gloss Roughness Viscosity of coating Ex. rpm m³/h kg/h d 4.3 d 10 d 50 d 90 μm 60° 85° Sheen RZD Ra s μm 2a 11000 150 20 6.49 3.74 5.95 9.7 23 16.6 66.4 49.8 2.24 0.28 36 40 2b 11000 150 40 12.9 3.69 6.68 24.3 23 21.9 58.0 36.1 2.00 0.24 39 39 2c 10000 150 20 11.5 4.99 8.47 17.9 27 16.6 58.8 42.2 3.24 0.42 2d  8000 150 30 12.2 5.76 11.5 19.5 39 15.6 43.8 28.2 4.30 0.55 36 42 2e 11000 150 30 7.6 3.55 6.1 12.44 24 21.1 55.4 34.3

TABLE 3 Classifying precipitated silica, prepared according to example 1c Classifier Through- Thickness Speed air put Particle size (Malvern) Grindo Gloss Roughness Viscosity of coating Ex. Fraction rpm m³/h kg/h d 4.3 d 10 d 50 d 90 μm 60° 85° Sheen RZD Ra s μm 3a fine 16000 53 4.3 7.42 4.24 6.78 11.13 22 25.3 75.7 50.4 23 30 coarse 12.07 8.05 11.28 16.99 33 12.1 27.6 15.5 21 30 3b fine 16000 66 2.0 6.84 3.95 6.30 10.11 23 26.2 74.9 48.7 23 30 coarse 11.18 8.26 10.93 14.45 33 12.3 26.4 14.1 21 30 3c fine 13000 117  6.0 7.42 4.24 6.82 11.07 22 23.1 71.9 48.8 2.13 0.26 23 30 coarse 11.08 8.03 10.73 14.48 33 13.9 35.6 21.7 21 30 Classifying precipitated silica, prepared according to example 2c Speed of Milling Through- classifier air put Particle size (Malvern) Grindo Gloss Roughness Viscosity Ex. Fraction Yield rpm m³/h kg/h d 4.3 d 10 d 50 d 90 μm 60° 85° Sheen RZD Ra s 4a fine 85 13000 2.1 6.84 3.95 6.26 10.10 29 19.8 70.3 50.7 2.2 0.27 26 coarse 15 10.17 8.32 9.91 12.35 29 10.9 31.2 20.3 24 4b fine 66 16000 2.1 7.37 3.01 4.84 11.08 17 21.8 77.6 55.8 26 coarse 34 9.36 8.45 9.28 10.4 27 10.5 36.2 25.7 24

TABLE 4 Particle size (μm) Grindo Gloss d 4.3 d 10 d 50 d 90 μm 60° 85° Sheen Com- 18.7 6.4 14.9 35.1 >100 10.5 15.2 4.7 parison example 4 Com- 12.8 3.4 6.2 20.7    85 18.4 62.4 44.0 parison Specks, example 5 air bubbles

TABLE 5 CC lacquer DE 38 NIPSIL Example according to: 15 670 1a 1b 1c E 1009 Flowtime in DIN seconds at 140 149 148 135 118 23° C. Thickness of coating in μm 23 23 24 23 23 60° reflectometer value 36.9 36.7 36.3 37.7 44.4 (DIN 67530) 85° reflectometer value 79.3 78.9 77.7 77.5 86.5 (DIN 67530) Sheen 42.4 42.2 41.4 39.8 42.1

TABLE 6 DD lacquer DE 38 NIPSIL Example according to: 15 670 1a 1b 1c E 1009 Amount of matting agent added 7.5 7.5 7.5 7.5 7.5 Flowtime in DIN seconds at 31 42 41 32 23 23° C. 60° reflectometer value 19.5 30 30.2 43.7 90.4 (DIN 67530) 85° reflectometer value 55.6 68.1 68.2 74.9 97.5 (DIN 67530) Macbeth RD 918 densitometer 2.12 2.31 2.17 2.16 2.3 value measured using yellow filter

EXAMPLE 7

The matting efficiency is determined in a number of different lacquer systems, wherein the preparation and application of the lacquer took place under identical conditions each time.

A high matting efficiency means a low requirement (concentration) of matting agent in order to achieve a specific degree of gloss (measured at an angle of 60°). The matting efficiency of unknown matting agents is determined in a relative manner, i.e. by comparison with known matting agents, so that variations in the determination of the degree of gloss (depending on the mode of preparation and application of the lacquer) are avoided. One important physico-chemical parameter which has a critical effect on the matting efficiency of silica is the particle size distribution of the silica. Basically, it has been shown that with identical precipitation processes the matting efficiency of the precipitated silica decreases with decreasing particle size (and vice versa). Fine fractions of precipitated silica have a lower matting efficiency than that of a more coarsely milled fraction.

The high matting efficiency of the precipitated silicas accoding to the invention is demonstrated as follows, in a variety of lacquer systems:

Table 7: Test in Alkyd/melamine Lacquer

Lacquer system: Alkyd Melamine in Accordance with Formulation

Product from example 2c has higher matting efficiency than Syloid ED 5, although this product is more finely divided. Furthermore, product 2a is more efficient than Nipsil E and Syloid ED

Particle Particle Particle Product prepared Weight Particle size size size Grind- RZD Ra Thickness according to added size d10 d50 d90 ometer Gloss Gloss roughness roughness Viscosity of coating example g d4.3 μm μm μm μm 60° 85° Sheen (A/M) (A/M) s μm 1 + 3 4 12.32 6.58 11.48 18.83 32 16.0 43.0 27.0 3.43 0.46 34 32 1 + 3 4 11.85 5.99 10.90 18.70 34 16.0 46.0 30 37 32 2 4 12.22 5.76 11.53 19.50 40 16.4 45.0 28.6 4.30 0.55 36 42 OK 520 4 7.20 31 16.5 64.0 47.5 3.05 0.36 38 37 2 4 11.50 4.99 8.47 17.97 30 16.6 56.8 40.2 3.24 0.42 36 38 2 4 10.90 5.55 10.41 16.46 37 16.9 47.8 30.9 38 27 1 4 13.24 6.42 12.90 20.40 33 17.8 43.6 25.8 36 30 1 + 3 4 12.32 6.58 11.48 18.83 33 17.9 50.2 32.3 3.43 0.46 33 40 Syloid ED 5 4 10.47 6.30 9.56 16.82 32 18.7 51.0 32.3 3.65 0.46 32 41 1 + 3 4 8.85 4.50 8.37 13.19 25 19.8 61.9 42.1 2.80 0.35 37 32 1 + 3 4 8.85 4.50 8.37 13.19 25 21.0 63.0 42.0 34 40 1 4 11.37 5.81 10.95 17.12 34 21.5 55.2 33.7 35 28 1 4 7.10 27 21.8 70.3 48.5 2.37 0.28 36 Syloid ED 3 4 6.04 3.62 5.54 8.88 21 22.0 73.0 51.0 2.03 0.24 35 34 Nipsil E 1009 4 7.92 4.34 6.97 12.51 27 22.0 70.0 48.0 2.44 0.28 38 32 OK 607 4 4.60 4.20 18 22.5 78.5 56.0 1.70 0.20 35 32 2 + 3 4 6.84 3.95 6.26 10.10 22 22.9 74.6 51.7 2.20 0.27 35 39 2 4 12.47 4.03 7.17 29.37 27 23.1 74.1 51.0 2.08 0.26 34 41 1 4 8.34 4.48 7.03 12.89 23 23.8 72.0 48.2 2.27 0.27 36 30 1 4 10.10 5.03 7.80 14.71 23 24.1 70.7 46.6 36 30 1 4 8.52 4.84 7.57 12.94 23 24.4 71.0 46.6 38 30 1 4 9.34 4.52 8.03 13.87 28 24.7 67.9 43.2 34 28 1 + 3 4 7.42 4.24 6.82 11.07 24 25.0 73.0 48.0 2.13 0.26 38 34

TABLE 8 Tests in DD lacquer Lacquer system: DD lacquer in accordance with formulation Comparison example: Syloid ED 3 Malvern Particle Particle Particle Grind- Densi- Rough- Rough- Weight value size size size ometer tome- ness ness Viscos- Thickness Product added d4.3 d10 d50 d90 (A/M) ter Gloss Gloss RZD Ra ity of coating Lacquer ref. g μm μm μm μm μm value 60° 85° Sheen (A/M) (A/M) s μm system 2b 7.65 12.93 3.69 6.68 24.35 25 2.11 25.0 66.2 41.2 2.00 0.24 n. m. ca. 40 DD 2d 8.00 12.22 5.76 11.53 19.50 40 2.16 24.7 40.3 15.6 4.30 0.55 32 ca. 40 DD 3c 8.2 7.42 4.24 6.82 11.07 22 2.12 25.0 65.6 40.6 2.13 0.26 53 ca. 40 DD 2a 8.24 6.49 3.74 5.95 9.70 24 2.11 24.5 59.7 35.2 2.24 0.28 55 ca. 40 DD 1a 8.41 8.34 4.48 7.03 12.89 25 2.08 25.0 60.9 35.9 2.27 0.27 n. m. ca. 40 DD Precip. 10.1 7.83 4.67 7.17 11.56 23 2.01 25.0 61.9 36.9 1.95 0.24 53 ca. 40 DD silica Syloid 10.7 6.04 3.62 5.54 8.88 21 2.24 25.0 68.2 43:2 2.03 0.24 52 ca. 40 DD ED 3

TABLE 9 Tests in DD lacquer Lacquer system: DD lacquer in accordance with formulation Comparison example: Nipsil ED 1009 Particle Particle Particle Particle Grind- Densi- Rough- Rough- Weight size size size size ometer tome- ness ness Viscos- Thickness Product added d4.3 d10 d50 d90 value ter Gloss Gloss RZD Ra ity of coating Lacquer ref. g μm μm μm μm μm value 60° 85° Sheen (A/M) (A/M) s μm system 2b 7.65 12.93 3.69 6.68 24.35 25 2.11 25.0 66.2 41.2 2.00 0.24 n. m. ca. 40 DD 1a 8.41 8.34 4.48 7.03 12.89 25 2.08 25.0 60.9 35.9 2.27 0.27 n. m. ca. 40 DD Nipsil 11.3 7.92 4.34 6.97 12.51 27 1.96 25.0 60.5 35.5 2.44 0.28 35 ca. 40 DD E 1009

TABLE 10 Tests in Coil coating lacquer Lacquer system: coil coating lacquer in accordance with formulation Particle Particle Particle Particle Weight size size size size Grind- Product prepared added d4.3 d10 d50 d90 ometer Gloss Gloss Viscosity according to example g μm μm μm μm μm 60° 85° Sheen s HK 125 2.7 4.9 9.65 17.35 30 24.0 45.0 21.0 95 Syloid C 812 2 6.40 12.50 20.80 40 27.0 44.0 17.0 90 1 2 12.36 6.20 11.33 19.31 32 27.0 48.0 21.0 101  1 2 14.56 6.82 13.31 23.30 40 28.0 48.0 20.0 102  Lovel HSF 2 6.74 13.22 22.96 44 29.0 42.0 13.0 77

TABLE 11 Test in an acrylic dispersion (aqueous) Lacquer system: acrylate dispersion (MB 2399-134), aqueous, from the Rohm and Haas company Comparison product: AQ 75 N Weight added Grindometer Densitometer Product name g μm value Gloss 60° Gloss 85° Sheen TS 100 (Commercial 0.25 41 2.5 69.3 92.3 23.0 product from Degussa AG) TS 100 (Commercial 0.5 41 2.4 56.1 87.0 30.9 product from Degussa AG) TS 100 (Commercial 0.75 41 2.28 44.7 82.0 37.3 product from Degussa AG) TS 100 (Commercial 1 41 2.17 30.4 73.4 43.0 product from Degussa AG) Precipitated silica 1 29 2.09 31.3 53.8 22.5 according to example 1b AQ 75 N (Commercial 1 28 1.95 39.0 68.2 29.2 product from Crosfield) Precipitated silica 1.5 29 1.89 18.1 35.2 17.1 according to example 1b TS 100 (Commercial 1.5 41 1.82 18.7 59.5 40.8 product from Degussa AG) AQ 75 N (Commercial 1.5 28 1.91 31.9 61.0 29.1 product from Crosfield) Precipitated silica 2 29 1.79 12.4 25.2 12.8 according to example 1b TS 100 (Commercial 2 41 1.8 15.3 66.0 50.7 product from Degussa AG) AQ 75 N (Commercial 2 28 1.89 27.7 53.3 25.6 product from Crosfield) AQ 75 N (Commercial 2.5 28 1.87 21.3 51.5 30.2 product from Crosfield) AQ 75 N (Commercial 4 28 — 12.2 35.8 23.6 product from Crosfield)

Particle sizes are determined using a laser beam diffractometer from the Malvern company. Before the measurement, the silica is dispersed in water using a stirrer and ultrasound. This silica dispersion is then pumped round the instrument into the path of the beam using a pump.

Sheen is the difference in the degree of gloss measured at an angle of 85° and the degree of gloss measured at an angle of 60°.

The viscosity is determined using a 4 mm DIN cup. The flow time in seconds of the lacquer is measured in accordance with DIN 53 211.

Key to the abbreviations: CC lacquer: coil coating lacquer DD lacquer: Desmodur Desmophen lacquer Desmodur is a hardener based on isocyanates Desmophen is a polyalcohol, used as the binder component Desmodur/Desmophen are the registered trade names of Bayer AG CAB cellulose acetobutyrate A/M alkyd/melamine lacquer

EXAMPLE 8

Coating with Polyethylene Wax Emulsion.

Precipitated silica is prepared according to DE-OS 31 44 299, Example 1. A wax emulsion (5% wax with respect to silica) is added to the filter cake which has been liquefied under the action of shear forces (solids content 10.8 wt. %) and then stirred vigorously for a further 30 minutes. The wax emulsion is prepared in an autoclave which is steam-heatable and has a disperser. 4.8 parts by weight of an alkylpolyglycol ether (Marlowet® CFW) in 81.0 parts by weight of water at about 100° C. is initially introduced. Then 14.2 parts by weight of low pressure polyethylene wax are added and heated to 130° C. On reaching 130° C., the disperser is switched on and dispersion takes place for 30 minutes. During this time the temperature is held at between 130° C. and 140° C. After switching off the disperser and cooling to about 110° C., the final emulsion is discharged.

The polyethylene used is characterized by the following properties:

Average molecular weight 1000 Solidifying point 100-104° C. Dropping point 110-117° C. Density (g/cm³) 0.93

The silica suspension coated with wax in this way is then dried in a rapid dryer (e.g. a spray drier) by atomizing (e.g. two-fluid nozzle, 2.8 bar of atmospheric air). The dried product is milled in a mechanical classifier mill of the ZPS 50 type from the Alpine company. The physico-chemical data are given in table 12:

TABLE 12 8a 8b N₂ surface area m²/g 373 373 CTAB-surface area m²/g 333 333 DBP absorption g/100 g 330 330 C content % 3.4 3.4 PH 7.2 7.2 Compacted density g/l 106 87 Particle size distribution (Malvern) in μm d₉₀ 26.25 12.28 d₅₀ 14.85 8.21 d₁₀ 6.91 4.66

TABLE 13 Alkyl melamine lacquer Comparative Example *) 8 a 8 b OK 500 OK 520 Flow time in DIN - seconds at 23° C. 31 29 30 32 Grindometer value   μm 41 26 25 28 Thickness   μm 30 29 29 28 60° -Reflectometer value (DIN 67530) 11.0 17.3 19.0 21.0 85° -Reflectometer value (DIN 67530) 24.3 42.9 69.5 76.9 Sheen 13.3 25.6 50.5 55.9 *) Degussa commercial product

TABLE 14 DD lacquer Comparative Example *) 8 a 8 b OK 500 OK 520 Flow time in DIN - seconds at 23° C. 23 27 29 30 Weight of matting agent added (g) 8.5 8.5 8.5 8.5 60° -Reflectometer value (DIN 67530) 21.6 34.4 69.9 8.6 85° -Reflectometer value (DIN 67530) 33.2 67.4 88.2 32.5 Sheen 11.6 33.0 18.3 23.9 Densitometer value - Macbeth RD 918 2.12 2.32 2.31 1.69 measured using yellow filter *) Degussa commercial product

While the invention has be described above with respect to certain embodiments thereof, variations and modifications may be made without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A precipitated silica matting agent coated with a polyethylene wax emulsion, characterized by the following physico-chemical parameters: BET surface area (DIN 66131) in m²/g 351-600 DBP index (DIN 53601) as a % 300-380 Carbon Content as a % 1-8 Compacted density (DIN 53194) in g/l  70-140 Grindometer value (ISO 1524) in μm 26-41 d₁₀ value 4.66-6.91 Size distribution index I 0.464-0.65  Measured with a Malvern instrument, wherein ${{Size}\quad {distribution}\quad {index}\quad I} = {\frac{d_{90} - d_{10}}{2d_{50}}.}$


2. A process for preparing a precipitated silica matting agent coated with a polyethylene was emulsion in accordance with claim 1, comprising: adding a polyethylene wax emulsion to a precipitated silica having the following characteristics: BET surface area (DIN 66131) in m²/g 400-600 DBP index (DIN 53601) as a % 340-380 Compacted density (DIN 53194) in g/l 180-220 “Alpine” sieve residue > 63 μm wt. % 25-60

to form a mixture; drying the mixture; and milling the dry mixture using a classifier mill or a fluidized bed counter-flow mill.
 3. The process according to claim 2, wherein the drying step is carried out by spray drying.
 4. A process for using the precipitated silica according to claim 1, comprising: adding the precipitated silica to a lacquer system as a matting agent. 